A top separator is a conventional device in a continuous digester that separates cellulosic fibrous material in a slurry (typically from a high pressure feeder) from some of the liquid of the slurry, and returns the separated liquid to upstream equipment (such as the high pressure feeder). Examples of top separators are shown in U.S. Pat. Nos. 5,413,677; 6,024,837; and 6,086,717 the disclosures of which are hereby incorporated by reference herein.
Conventional top separators 10 are mounted on the top of a digester vessel 16 and are sealed by a packing 12 around a top separator shaft 14 to the digester. The packing seals the pressurized slurry in the digester vessel. Packing is usually a lubricated series of rings, such as five rings. From time to time, the packing needs to be replaced or refurbished in order to maintain a good seal of the pressurized digester vessel.
A problem with the current design is that the digester vessel must be emptied of the slurry to refurbish or replace the packing 12. Emptying the digester is a time-consuming and expensive proposition. FIG. 1 shows a side schematic view, partly in cross-section and partly in elevation, of a conventional packing for a top separator. FIG. 2 is a view like that of FIG. 1, showing the conventional packing and its associated pack box 18 in greater detail.
The pack box 18 is an annular chamber around the shaft 14 that contains the packing 12. Lubricant may be added to the packing chamber and packing by a lubricant port 19 and through a lantern ring 21.
In one embodiment, the invention is a top separator assembly for a digester comprising a rotatable shaft for the top separator; a packing for the shaft, and a sealing ring operatively mounted to the shaft and having a nominal clearance with the pack box. The sealing ring is mounted so that up-lifting of the shaft (while the shaft is not rotating and during digester maintenance) provides a substantially fluid-tight seal between the pack box and a side of the ring (in the digester) opposite the pack box. This fluid-tight seal by the sealing ring maintains the pressure of the contents of the digester vessel while the packing is replaced or refurbished.
In another embodiment, the invention is a top separator assembly for a digester, comprising: a rotatable shaft for the top separator assembly; a pack box including packing mounted around the shaft; and a sealing ring mounted on the shaft and having a nominal clearance with the pack box, said sealing ring mounted so that up-lifting of said shaft provides a substantially fluid-tight seal between said pack box, said ring and the digester.
In a further embodiment, the invention is a method for servicing packing material in a top separator assembly of a digester, said method comprising the steps of: lifting a shaft extending from the top separator assembly and into the digester; while lifting the shaft, creating a fluid-tight seal between a sealing ring mounted on the shaft and a pack box housing the packing material; refurbishing the packing material while maintaining a slurry in the digester under pressure using the seal formed by the sealing ring; lowering the shaft such that the packing material forms a seal between the pack box and shaft, wherein the lowering of the shaft forms a clearance between the sealing ring and the pack box, and rotating the shaft during operation of the digester, wherein the shaft is sealed by the pack box and the sealing ring is separated from the pack box.